Many industries, such as the pharmaceutical industry, employ sensing devices to monitor the progress or outcome of a particular event. Often, these sensing devices are used to provide input to a controller, which then varies its output, based on this input. The output of the controller is typically used to influence, affect or control the particular event.
For example, in a bioreaction, it may be important to monitor and control a number of characteristics of the reaction, including but not limited to temperature, pH, oxygen concentration, or other parameters. Because of this, devices have been developed to sensor these characteristics. There exists a plethora of pH sensors, dissolved oxygen sensors and temperature sensors.
However, while sensing the characteristic is important, it is equally important to be able to monitor and track these characteristics over time. Additionally, it is important to use these characteristics to determine future actions. For example, FIG. 1 shows a simple example of a closed loop control system which can be used with a bioreactor. In this figure, a temperature sensor 10 may be used to monitor the temperature of a particular reaction occurring within a reaction chamber 20. Based on the output of the temperature sensor 10, a controller 30, in communication with that sensor 10, may vary the output of a heating element 40. In this way, if the temperature within the reaction chamber must be within a prescribed temperature range, the controller 30 can use the sensor 10 and the heating element 40, in conjunction with a software control loop, to insure that these conditions are met.
Additionally, the sensor 10 may be in communication with other devices. For example, the output of a pH sensor may be in communication with controller, a logging device and/or data storage device. The attached device samples the output of the specific sensor over time. This sampling step may be performed periodically, such as at fixed time intervals. In other embodiments, this sampling step is performed at sporadic intervals, or based on other external events.
In the case of a data logger, the value sampled is simply stored, usually with an associated timestamp, so that a graph of that characteristic over a period of time can be generated. In the case of the controller, the value is sampled so that corrective action can be performed. For example, as described above, in the case of a temperature sensor, the controller may be in communication with a heating element, such that if the temperature reading is below a predetermined threshold, the controller actuates the heating element. Similar actions can be taken in response to pH or dissolved oxygen readings.
One of the most common dissolved oxygen sensor is known as a polarographic sensor. A representative sensor is shown in FIG. 2. The sensor 100 includes a membrane 120 through which oxygen can pass. It also includes a cathode 130 and an anode 160. In some embodiments, the cathode 130 is made from a conductive material, such as platinum. In some embodiments, the anode 160 is made from a conductive material, such as gold or silver. These two conductive components are separated by an insulator 140, such as glass. An external device provides a voltage potential, such as between 600 and 800 mV, between the cathode 130 and the anode 160.
In operation, oxygen molecules diffuse through the membrane 120. These molecules are reduced at the surface of the cathode 130, such as according to the following equation:O2+2H2O+4e−→4OH−At the anode, an oxidation reaction is occurring, thereby producing electrons. These electrons move toward the cathode 130, thereby generating a current proportional to the oxygen concentration. This current can then be measured by a device, such as a controller or a logging device.
Because of the popularity of polarographic sensors, many devices, such as controllers, including those made by Applikon, were designed to interface directly to them. In other words, these devices, provided a polarizing voltage of 600-800 mV, and were designed to measure the resulting current flow, which is in the range of 0-100 nA. These devices also were used to control operations, such as bioreactions, and have been used for a significant amount of time. Thus, there exists a large installed base of these controllers and other devices, configured to interoperate with polarographic sensors.
More recently, alternative sensors have been developed. Unlike traditional polarographic sensors, these alternative sensors typically use a different indicator of oxygen content. One such indicator is fluorescence. In one embodiment, the sensor has an emitter, which emits light, typically at a specific wavelength, such as 475 nm. The light is directed toward a sensing element. The sensing element has a thin layer of hydrophobic material. A compound capable to fluorescing, such as ruthenium, is trapped within the hydrophobic material, effectively shielded from the water. The light excites the ruthenium, which then emits energy at a specific wavelength, such as 600 nm.
Oxygen is able to effectively quench the fluorescence of ruthenium. Collision of an oxygen molecule with a fluorophore in its excited state leads to a non-radiative transfer of energy. Thus, the more collisions that occur, the less fluorescence is created. The frequency of collisions is directly related to the concentration of oxygen molecules. Therefore, the measured fluorescence is a direct measure of the concentration of oxygen molecules.
These sensors are typically more accurate than traditional polarographic sensors. Furthermore, since they do not include any precious metals, such as platinum and silver, they are typically much less expensive. These qualities make these newer sensors the preferred choice in many applications. For example, disposable systems are more likely to utilize fluorescence oxygen sensors, due to the lower cost (especially when taking into account that the sensor will be discarded with the bag).
However, today, it is not possible to use these new sensors with existing systems. Unlike polarographic sensors, this optical-based sensors do not require a polarizing voltage input. Furthermore, rather than producing a very small current, the output of an optical-based oxygen sensor is typically between 0 and 10 volts. In another embodiment, the output is typically between 4 and 20 mA. These outputs are completely incompatible with the input characteristics of existing devices, such as Applikon controllers. Therefore, the adoption of these new optical-based sensors has been slowed.
Therefore, it would be beneficial if there were a system and method whereby these new, inexpensive, accurate oxygen sensors can be employed with existing devices, such as data loggers and controllers. Furthermore, it would be advantageous if these sensors were compatible with both older legacy controllers, and newer devices, such that two devices, such as a data logger and a controller, can be used simultaneously.